1. Field of the Invention
The present invention relates to a resistance type sensor suitably used for detecting a force applied from outside.
2. Description of the Related Art
A resistance type sensor is in general use as a device for converting magnitude and direction of a force applied by an operator into electric signal. For example, a device having the resistance type sensor for inputting operation of multidimensional direction incorporated as a so-called joystick is used as an input device of a mobile phone.
A resistance type sensor can be used to input an operation having a specified dynamic range as a magnitude of a force applied by an operator. It is also used as a 2-dimensional or 3-dimensional sensor capable of dividing an applied force into dimensional components, for detection of the applied force.
For example, Japanese Laid-open (Unexamined) Patent Publication No. Hei 6(1994)-347850 filed by the applicant of this application discloses a resistance type sensor having a pressure-sensitive member interposed between a pair of electrodes. It discloses a technique of detecting magnitude of the force on the basis of changes of the resistance value between the pair of electrodes when the operation of the sensor is performed (when a force from outside is applied to the sensor).
In this resistance type sensor, since electric current flows through the circuit, regardless of the operational state of the sensor, electric power is always consumed. Preferably, when operation is not applied to the resistance type sensor for a long time, the flow of the electric current through the sensor circuit should be stopped by taking appropriate measures.
For example, in the case where an input device having a switch function of selectively switching between the on-state and the off-state is used with a microcomputer control system, it is general that when the input device is not operated (when no switching operation is performed) after passage of a predetermined time, the resistance type sensor is automatically switched to a sleep mode (power-saving mode) in which power consumption is reduced as much as possible. The sleep mode is not canceled automatically until the input device is operated. An output from the input device is either of signal at a high level near a power-supply voltage and signal at a low level near a ground potential. When the input device is operated, the output varies in level, switching from a low level to a high level or from a high level to a low level. This means that in this input device, the output after operation surely varies across a threshold voltage which is about half the power-supply voltage from its initial state before operation. Thus, the operation applied can be surely detected by monitoring the output from this input device and thereby the sleep mode can be properly cancelled.
This resistance type sensor is suitably used for a device that can detect magnitude of a force applied from outside (e.g. a force sensor), but is not suitable for a device having the switch function of selectively switching between two different modes (e.g. on-state and off-state). Accordingly, when this resistance type sensor is built in equipment as a device having a multidirectional switch function, the resistance type sensor can hardly be used as it is and is required to add switch functions corresponding to the respective directions.
In addition, in this resistance type sensor, the output from the sensor may not vary across the threshold voltage depending on magnitude of a force applied from outside. In this case, the operation applied cannot surely be detected by simply monitoring the output from the sensor. Due to this, in the case where the resistance type sensor is automatically switched to the sleep mode when the input device is not operated after passage of a predetermined time, as in the case of the input device having the switch function mentioned above, the sleep mode may not be canceled properly. Thus, even when the input device is not operated for a long time, this resistance type sensor may not be switched to the sleep mode, so that the electric power is consumed wastefully during that time.